1. Field of the Invention
This invention relates generally to a method and apparatus for assessing baroreflex sensitivity, and particularly to a method and apparatus for establishing an optimum pacing site and AV delay parameter for a programmable pacemaker.
2. Discussion of the Related Art
Congestive heart failure is an insidious disease affecting at least two million Americans. Patients diagnosed with heart failure have an extremely poor long-term prognosis. The average life expectancy of a person suffering from chronic heart failure is now only five years. Because of the severity of chronic heart failure, a need exists for developing efficacious therapies for this disease. The potential market for an efficacious therapy is not only large but also highly motivated. The patient""s clinical and hemodynamic status determines the baroreflex sensitivity (BRS) that is a measure of the ability of that individual""s heart to react to changes in blood pressure by changing heart rate. The BRS of a chronic heart failure patient parallels that patient""s clinical and hemodynamic status. Thus, BRS provides an indicator for the efficacy of a drug therapy or a ventricular resynchronization therapy. BRS may also be used to predict arrhythmic events and mortality in these patients.
Current methods for measuring BRS typically require drug infusion or a Valsalva""s maneuver. Drug infusion, typically utilizing phenylephrine, is used to chemically induce vasoconstriction that changes the pulse pressure. The change in pulse pressure induces a change in the heart""s cycle length. Both the change in pulse pressure and the change in cycle length are measured relative to established baselines. The two coordinates are plotted on a graph. Repeated administration of the drug develops a regression line having a slope that is a measure of BRS. Similarly, the Valsalva""s maneuver mechanically induces a change in pulse pressure to measure BRS to reach the same result as drug infusion. Neither drug infusion nor the Valsalva""s maneuver is timely, and the results from measurement using the Valsalva""s maneuver are typically less repeatable and hence less reliable than desired for medical applications. Further, drug infusion and the Valsalva""s maneuver cause significant patient discomfort and are potentially dangerous to the patient. Therefore, a need exists for a fast, comfortable and reliable method to determine BRS in chronic heart failure patients.
Drug and pacing therapies are commonly used to extend the lives of chronic heart failure patients. With pacing therapies, pacing site and atrioventricular (AV) delay are critical to the efficacy of the therapy. In early pacing devices, the AV delay was not adjustable to fit a patient""s changing condition after implantation. Later devices permitted the external reprogramming of the AV delay by the physician, although the parameters evaluated for reprogramming still required the use of invasive techniques to measure.
Other devices utilize heart rate variations associated with changes in the paced AV delay measured over a sufficient time interval that transient (short-term) variations can be ignored and only the steady state variations assessed. Still other devices provide for optimizing both the pacing mode and the AV delay parameter by incorporating a sensor to provide an average value of a physiologic parameter over a predetermined time interval for each change in pacing mode/AV delay interval. The particular AV delay associated with an optimum value for the physiologic parameter is used to establish the optimum AV delay until the algorithm is again executed and a new optimum AV delay is established. Although studies indicate the efficacy of the above therapies, none of the therapies utilize the patient""s BRS in determining the appropriate therapy. Given the strong correlation between BRS and a patient""s clinical and hemodynamic status, there is need for a method of optimizing therapy based on BRS.
The present invention meets the above needs and provides additional advantages and improvements that will be evident to those skilled in the art.
The present invention provides an apparatus and method for measuring the BRS of a patient having an implanted pulse generator. In addition the present invention provides a method for determining a therapy""s efficacy. The apparatus for measuring baroreflex sensitivity includes a pacemaker, a first sensor and a second sensor. The pacemaker has at least one variable parameter capable of altering a pulse pressure in a patient. The pacemaker may be of the type having atrial and ventricular depolarization sensors, and capable of selectively stimulating the right, the left or both ventricular chambers. The pacemaker typically provides pacing pulses with a selected AV delay following detection of atrial depolarization events. The AV delay may be pre-selected or altered after implantation of the pacemaker. While cycle length is typically described as atrial cycle length, it should be recognized that ventricular cycle length could also be the cycle length used in determining BRS. The pacemaker typically includes a processor receiving data from the first and second sensors and controlling the pulse generator of the pacemaker. The first sensor senses the cycle length of the patient""s heart. The second sensor senses the patient""s pulse pressure. The first sensor may be selected from the group of electrocardiographic sensors, an atrial electrographic sensors and a ventricular electrographic sensors, among others. The second sensor may be selected from the group of Millar pressure transducers, RV pressure transducers, LV pressure transducers, RV impedance sensors, LV impedance sensors, aortic cuffs, accelerometers, echo catheters, Doppler echo devices, external pressure cuffs, external impedance sensors, radial tonometers, and plethysmogram sensors, among others. The apparatus may include a visual display connected to the pacemaker to display data indicative of the baroreflex sensitivity. The apparatus visual display may be connected to the pacemaker by radio frequency, a wire, or otherwise as is recognized by those skilled in the art. The apparatus may also include an external programmer. The programmer typically has an external processor and a display. The programmer may display data indicative of BRS calculated by the pacemaker""s processor or the programmer may include an external processor that receives information from data from the pacemaker""s processor and displays data indicative of baroreflex sensitivity on the display. The apparatus may include an implanted transmitter connected to the processor implanted in the patient and a receiver connected to the external processor in the external programmer wherein the implanted transmitter transmits data indicative of baroreflex sensitivity to the receiver. Similarly, the apparatus may include an implanted receiver connected to the processor implanted in the patient and a transmitter connected to the external processor in the external programmer wherein the transmitter transmits data to the receiver to alter the pacing parameters.
The method of measuring BRS in accordance with the present invention uses the pulse generator in the pacemaker to alter the pulse pressure of the patient. This method produces a measurement of BRS analogous to previous methods for BRS measurement and therefore allows for direct comparison. This measurement of pulse pressure also provides an immediate indicator of the efficacy of the particular pacing mode being applied compared with the pulse pressure of the heart""s intrinsic rhythm. The method of the present invention, includes the steps of pacing the heart, measuring the changes in pulse pressure and intrinsic cycle length, and calculating the patient""s baroreflex sensitivity from these measured parameters. The change in pulse pressure are for purposes of the present invention the difference between a baseline pulse pressure and the actual pulse pressure during pacing. The change in cycle length are for purposes of the present invention the difference between a baseline cycle length and the cycle length induced by the change in pulse pressure. The pacing step includes pacing a heart for a plurality of paced heart beats, wherein each of a pacing series are followed by a plurality of intrinsic heart beats. The plurality of paced beats defined as the pacing series and the plurality of intrinsic heart beats defined as the intrinsic series. The pulse pressure measurement step measures the change in pulse pressure during the pacing series. The pulse pressure may be measured using a method selected from Doppler echo, radial tonometry, thermodilution, dye dilution, MUGA, plethysmography, Fick method, acetylene rebreathing technique and carbon dioxide rebreathing technique. The intrinsic cycle length measurement step measures the change in intrinsic cycle length resulting from the change in pulse pressure during the pacing series. The measurements of these changes may be made during at a quiet time where the patient is at rest. The baroreflex sensitivity is then calculated from the change in pulse pressure measurements and the change in intrinsic cycle length measurements after a plurality of pacing series.
This calculation of baroreflex sensitivity may be used to evaluate the efficacy of therapies and to select the most efficacious therapy that is the therapy producing the highest baroreflex sensitivity. For this application the baroreflex sensitivity is measured before the cardiac rehabilitative therapy. The patient is then provided a cardiac rehabilitative therapy to the CHF patient. The patient""s baroreflex sensitivity is again measured after the cardiac rehabilitative therapy. The results of the therapy are then evaluated based on a comparison of the patient""s baroreflex sensitivity before the cardiac rehabilitative therapy and the patient""s baroreflex sensitivity after the cardiac rehabilitative therapy.
The foregoing features and advantages as well as other features and advantages of the present invention will become apparent to those skilled in the art upon review of the Detailed Description of the Invention, especially when considered in conjunction with the accompanying figures.